Regulating circuits that regulate the rotary speed of pulse-width-modulated fans are known. For example, DE 102004002447 B4 discloses a regulating circuit with overtemperature signaling. In that case, the fan regulating circuit contains an input that supplies a temperature-dependent control signal to provide a desired value for the rotary speed of the fan. When a first temperature threshold is reached, an overtemperature warning is signaled by inverting a signal at a first signal output which indicates the current rotary speed of the fan. This and other regulating circuits are particularly suitable to regulate electronically commutated fans which, on account of their connection diagram, are often also referred to as four-wire fans.
FIG. 1 illustrates an exemplary arrangement comprising a pulse-width-modulated fan 1, a regulating circuit 2 and a component 3 to be cooled. Arranged in the region of the component 3 is a temperature sensor 4 which can be used to detect the temperature of the component 3 to be cooled to predefine a regulation value for the regulating circuit 2. In the arrangement illustrated in FIG. 1, the components 2, 3 and 4 are arranged on a common printed circuit board 5 of a data processing device, for example, a computer system or a network component.
Conventional regulation of the fan 1 is described below using FIG. 2. The pulse-width-modulated fan 1 generates a tacho signal TACH, the tacho signal containing, for example, one or two tacho pulses per revolution of the fan wheel. This tacho signal is supplied to a frequency measuring device 6. The frequency measuring device 6 also receives a control signal from a timer device 7. The timer device 7 comprises, for example, a digital oscillator and a control logic unit to determine a window length to measure a frequency by the frequency measuring device 6.
The frequency measuring device 6 determines the number of tacho pulses in the tacho signal TACH which are detected in a time window determined by the timer device 7. For example, the frequency measuring device 6 can determine how many tacho pulses occur in one second. A rotary speed comparator 10 then compares this value NIst with a predefined desired value NSoll determined, for example, on the basis of a measured temperature θ.
The difference between the desired number of tacho pulses NSoll and the actual number of tacho pulses NIst is transmitted to a pulse-width control device 8 as a value ΔN. The difference value ΔN is then used to predefine a duty ratio as a digital value P to drive the fan 1 for a pulse-width modulation device 9 by a function p1(ΔN). The function p1 may be predefined, for example, in the form of a table containing control values P for particular difference values ΔN. A free-running counter of the pulse-width modulation device 9 is then readjusted to the desired duty ratio on the basis of the value P. The pulse-width modulation device 9 generates a pulse-width-modulated signal PWM therefrom and makes the signal available to the fan 1 via a drive line.
Detecting tacho pulses inside a limited measuring time window of one second, for example, reduces the accuracy with which the rotary speed is detected. A rotary speed of 800 revolutions per minute, for example, and two tacho pulses per revolution result in a counter reading of 26 after one second, for example, while the mathematically correct value is 26.67. In the previously described case, an error of approximately 2.5% thus already occurs when detecting the rotary speed.
The measurement could be improved, for example, by extending the time window, but the regulating speed of the regulating circuit 2 would then decrease. Furthermore, particular regulating parameters would additionally have to be complied with to prevent escalation of the fan rotary speed.
As described above, a free-running counter of the pulse-width modulation device 9 is then adjusted to the desired duty ratio on the basis of the determined number NIst of determined tacho pulses. Inexpensive counters of this type have a resolution of eight bits, with the result that a resolution of 0-100% of the duty ratio would be theoretically possible in 255 steps. However, since a sensibly usable frequency of the PWM drive signal for the fan 1 is often predefined within narrow limits, for example, 20-25 kHz, but an oscillator clock of the counter can generally only be divided down into steps of two and/or four, the uppermost possible control value of the pulse-width control device 8 must be limited to a value of lower than 255 to obtain the correct drive frequency. If this is effected to a value of 100, for example, a fan 1 having a maximum rotary speed of 5,000 revolutions per minute already only has a drive resolution of 50 revolutions per minute, depending on the usable bit of the control register. The increase or decrease in a rotary speed of 50 revolutions per minute is already clearly audible. Such audible changes in the operating noise of the fan 1 are often perceived as disruptive by a user of a data processing device.
The driving could be improved, for example, by a higher-quality pulse-width modulation device 9 having a counter with a higher resolution. However, this results in cost disadvantages since the counters present in integrated regulating circuits can then no longer be used.
It could therefore be helpful to describe an improved regulating circuit and a method of regulating the rotary speed of a pulse-width-modulated fan, which method improves the regulating accuracy. In this case, the method is intended to be able to be implemented as easily as possible using hardware or software.